This invention relates to a high pressure sodium vapor lamp utilizing alumina ceramic inner envelope and is particularly concerned with a high pressure sodium lamp having an inner arc tube of alumina ceramic operated at a relatively high pressure and having a desired color rendition which is maintained during the life of the high pressure sodium lamp.
High pressure sodium vapor lamps have found widespread use during the past decade for commercial lighting applications, especially outdoor lightings. Such lamps are described in U.S. Pat. No. 3,248,590-Schmidt, High Pressure Sodium Vapor Lamps. High pressure sodium lamps typically utilize a slender tubular inner envelope of transmissive refractory oxide material resistant to sodium at high temperatures, suitably high density polycrystalline alumina or electron emissive material. The ceramic arc tube is generally supported within an outer vitereous envelope or jacket provided at one end with the usual screw base. The electrodes of the inner envelope are connected to terminals of the base, that is to shell and center contacts. The space between the inner and outer envelope is typically evacuated in order to conserve heat.
The color rendition of standard high pressure sodium (HPS) lamps may be improved by increasing the internal sodium partial pressure within the arc chamber of the inner envelope formed of polycrystalline alumina. An improved color rendition of a standard HPS lamp may desire that the polycrystalline alumina arc chamber be operated with two to three times the internal sodium partial pressure relative to a conventional standard HPS lamp. In addition, for such increased sodium pressure the wall temperature between electrode tips within the arc chamber may be increased by 100.degree. C. relative to a standard HPS lamp in order to offset the reduction of luminous efficacy which normally accompanies operation at increased sodium pressures. Higher internal sodium pressures as well as higher arc chamber wall temperature typically shortens the life of improved color HPS lamps relative to their standard color, high efficacy HPS lamp counterparts. For example, manufacturers of improved color high pressure sodium lamps typically classify their products at 8,000-10,000 life hours as compared to 20,000-30,000 life hours for standard color products.
The life reduction of improved color HPS lamps typically occurs by the major factors of sodium losses from the arc chamber due to reactions with the inner surface of the arc chamber and through migration of the sodium though the walls of the arc chamber. Sodium losses result in HPS lamp voltage rise, decrease of correlated color temperature of the HPS lamp, decrease of color rendition index and a color shift from a desired warm white color to an undesired pink color. In addition, the sodium which migrates through the arc chamber deposits on the inside wall of the evacuated outer chamber where reaction with the glass and evaporation products from the alumina arc chamber cause a brownish stain on the outer chamber, which, in turn, reduces the light output of the HPS lamp. The brownish stain further aggravates undesirable lamp voltage rise of the HPS lamp.
Accordingly objects of the present invention are to provide means for reducing the loss of sodium from an improved color HPS lamp operated at relatively high pressures contributed by the sodium within the arc chamber of the HPS lamp, and to provide such a high pressure sodium lamp having a color rendition index which remains substantially constant over the life of the improved color HPS lamp.
These are the objects of the present invention and will become more apparent upon consideration of the following description of the invention.